The invention in general relates to body tissue stimulators, and more particularly to a cardiac pacemaker having one or more characteristics that can be varied by means of a magnetically actuated switch.
The art of implantable cardiac pacemakers was first disclosed by Greatbatch in U.S. Pat. No. 3,057,356 entitled "Medical Cardiac Pacemaker", which issued in 1962. The device disclosed by Greatbatch included a relaxation oscillator that generated electrical pulses at a fixed rate. These pulses were applied to the heart through a lead to cause the heart to contract each time a pulse occured.
Early in the development of the art of cardiac pacemakers it became evident that it was desirable to design pacemakers so that the parameters of the output pulse could be varied. An early means for varying the output pulse incorporated a reed switch. See, for example, U.S. Pat. No. 3,311,111 issued to D. L. Bowers and U.S. Pat. No. 3,518,997 issued to R. W. Sessions. The reed switch is a pair of magnetically susceptible and conductive reeds which maintain themselves separated by spring action unless they are merged together by magnetic forces. Generally the magnetic forces are applied by means of a magnet held external to the body in which the pacemaker is implanted. Generally the closing of the reed switch by the magnet actuates circuitry within the pacemaker which alters the output parameters according to some predetermined scheme. By this means the output parameters of an implanted pacemaker can be adjusted externally of the body.
Many different schemes which incorporate a reed switch have been developed for adjusting the output parameters of an implanted pacemaker externally to the body. See for example U.S. Pat. Nos. 3,623,486 issued to Barouh V. Berkovits; 3,631,860 issued to Michael Lopin; 3,738,369 issued to Theodore P. Adams and David L. Bowers; 3,805,796 issued to Reese S. Terry Jr. and Gomer L. Davies; and 4,066,086 issued to Clifton A. Alferness and John M. Adams. The latter patent describes the method most commonly used, in which the reed switch is employed to actuate a circuit for receiving radio frequency programming signals. In this manner a wide variety of output parameters can be varied using a single switch. Because of the above developments, physicians have become accustomed to using magnets to vary output parameters of cardiac pacemakers.
Although the reed switch has become almost universal in pacemakers it is recognized as having several disadvantages. The reed switch is generally the only mechanical element in the pacemaker and for this reason it is more susceptible to damage and breakage than the electronic elements which make up the rest of the pacemaker. In addition, in order to fit into a pacemaker, the switch must be made very small which adds to its fragility. Generally, the tiny reeds of the switch are encased in a glass capsule for protection, but the glass capsule itself is susceptible to being fractured.
The modern pacemaker is generally wafer shaped so that it may be placed underneath the skin of the body with a minimum bulge. A characteristic of the reed switch is that, in order for it to respond to a magnetic field, it must be placed along the thin axis of the pacemaker, or at least have a component of its length along this axis. Generally, pacemakers are now thinner than the length of a typical reed switch so that incorporating a reed switch within the pacemaker in an orientation in which it operates reliably has become increasingly difficult. One attempt to overcome the disadvantages of a reed switch has been described in U.S. Pat. No. 3,766,928 issued to Goldberg et al. This method incorporates a potentiometer which is affixed to a small diametrically magnetized disc magnet. A second magnet is rotated outside the body to cause the disc magnet to rotate and turn the potentiometer. This method itself has numerous disadvantages including the fact that it is also mechanical, and very small and thus prone to breakage. In addition, the manipulation of the second magnet in order to adjust the potentiometer is more difficult and complex than the manipulation of the magnet required to actuate a reed switch.
Another aspect of the prior art of the present invention is the technology of the Hall effect. The state of the art in this technology is described in "Magnetically Activated Monolithic Integrated Circuits For Analog And Digital Applications", by Robert A. Anaselmo and Michael H. Oppenheimer, technical publication 71-11 published by the Sprague Electric Company and available from Technical Literature Service, Sprague Electric Company, North Adams, Mass. 01247. Although magnetically activated monolithic integrated circuits have been commercially available for about ten years they have not, up to now, been incorporated into pacemaker technology. There are numerous reasons for this. Pacemakers necessarily must be small and in addition implanted pacemakers necessarily have a limited supply of electrical energy so the current source must be totally contained in the pacemaker. However, magnetically activated Hall devices, whether in monolithic integrated circuit form or in other form, require complex supporting circuitry and draw large currents. Thus, Hall effect technology has generally been rejected for pacemaker purposes with the result that this technology is relatively unknown among those practiced in the pacemaker art.